This invention relates to sensors for measurement of air/fuel ratio and more particularly to an air/fuel ratio measuring sensor which can detect the air/fuel ratio over a wide range covering low air/fuel ratio (rich region) and high air/fuel ratio (lean region) which are partitioned by the stoichiometric air/fuel ratio. The sensor is suitable for use in a fuel controller of an automobile engine.
In a fuel control system for automobile using an air/fuel ratio sensor, the actual air/fuel ratio in mixture gas is measured by detecting the concentration of oxygen and unburnt gases (CO, H.sub.2, HC) in the exhaust gas, and information indicative of the air/fuel ratio (hereinafter abbreviated as A/F) is fed back to a fuel flow control circuit which in turn controls the fuel flow such that the mixture gas maintains a target A/F. The A/F at which gasoline fuel in mixture gas is completely burnt is known as the stoichiometric A/F which is 14.7.
As present, a stoichiometric sensor (so-called O.sub.2 sensor) for detecting the stoichiometric A/F point or a lean sensor for continuously detecting the A/F over a range covering the stoichiometric A/F and the lean region is available as the A/F sensor.
Running automobiles in various ambient conditions and with widely changing loading, and therefore the A/F must be controlled adaptively over a wide range in accordance with running condition and load. For example, the A/F has to be controlled to lean A/F for light load, to rich A/F for heavy load and low temperature and to the stoichiometric A/F for a region where a 3-way catalyzer is activated.
However, technical difficulties are encountered in manufacture of a sensor which can afford to measure the A/F in a rich region because the structure of a gas diffusion layer of the sensor causes a bottleneck. The gas diffusion layer structure plays an important role in measurement of the rich A/F as will be described below with reference to FIGS. 1 to 3.
FIG. 3 shows the relation between A/F value and concentration of components in the exhaust gas. Excepting the five components shown in FIG. 3, nitrogen is predominent in the exhaust gas. In the lean region, nitrogen and oxygen are predominant as components in the exhaust gas and when compared with these components, carbon monoxide, hydrogen and hydrocarbon are contained by a very small amount in the exhaust gas. Contrarily, in the rich region, the amount of oxygen contained in the exhaust gas is small but carbon monoxide, hydrogen and hydrocarbon which stand for incompletely burnt components are predominant.
FIG. 1 shows a fragmentary section of an A/F sensor. In FIG. 1, 1 designates a solid electrolyte made of, for example, zirconia through which oxygen ions can permeate, 2a and 2b porous reaction electrodes across which a predetermined voltage V is applied, and 3 a porous gas diffusion layer made of, for example, ceramic.
In the case of the lean A/F, the oxygen concentration is higher on the exhaust gas side than on the reaction electrode 2b and consequently oxygen molecules move through the gas diffusion layer 3 and reach the reaction electrode 2b at which they are ionized. Oxygen ions O.sub.2.sup.- than pass through the solid electrolyte 1 to reach the electrode 2a. The amount of moving oxygen ions is detected to provide a pump current value Ip which is increased as the applied voltage increases. The gas diffusion layer 3 is so configured as to limit the diffusion speed of oxygen molecules passing through the layer 3. The diffusion speed depends on the size and number of pores present in the gas diffusion layer 3. When the diffusion speed of oxygen is properly limited, characteristics can be obtained wherein the current value Ip* is saturated over a certain voltage range as shown in FIG. 2. The saturated current value is called a diffusion limiting current value. The limiting current value changes with the A/F value. Accordingly, the A/F can be determined by detecting the limiting current Ip while keeping the voltage V constant.
In the case of the rich A/F, carbon monoxide, hydrogen and hydrocarbon which stand for incompletely burnt components in the exhaust gas diffuse through the gas diffusion layer 3. Oxygen in the atmosphere, on the other hand, is ionized while passing through the electrode 2a, and the oxygen ions move through the solid electrolyte 1 and combine with the carbon monoxide, hydrogen and hydrocarbon. Since the size of the molecule of the incompletely burnt components is far smaller than that of an oxygen molecule, the gas diffusion layer having ability to properly limit the diffusion speed of oxygen molecule can not limit the diffusion speeds of the incompletely burnt components, with the result that excessive amounts of incompletely burnt components migrate to the electrode 2b. On the other hand, the diffusion speed of oxygen prevailing on the atmosphere side is limited by a flow path resistance between the outer cap housing the sensor and the solid electrolyte. Further, because of low temperatures near the electrode 2b, oxidation (combustion) reaction of incompletely burnt components with ionized oxygen (O.sub.2.sup.-) at the electrode 2b is insufficient, thus preventing complete combustion. Accordingly, limiting current characteristics for the rich A/F can not be obtained and the rich A/F can not be detected. In order to realize detection of the rich A/F, the flow of excessive amounts of incompletely burnt components must be suppressed to decrease absolute value of the limiting current Ip*. To this end, a gas diffusion layer capable of limiting the diffusion speed of the incompletely burnt components is needed.
As well known in the art, the diffusion limiting current Ip* is expressed by the following theoretical formula (1): ##EQU1## where F: Faraday constant
R: gas constant PA1 T: aboslute temperature of gas PA1 S: equivalent sectional area of pores in the gas diffusion layer PA1 l: thickness of the gas diffusion layer PA1 .alpha.i: conversion constant PA1 Di: diffusion coefficient of molecules PA1 pi: partial pressure of gas. PA1 d: pore diameter PA1 T: temperature PA1 M.sub.A : molecular weight of A component molecule. PA1 V.sub.A : molecular volume of A component molecule. PA1 P.sub.A : partial pressure of A component molecule
The values of the limiting currents shown in FIG. 2 are determined by inserting values of constants and variables into equation (1). By unifying the constants, equation (1) can be reduced to equation (2), ##EQU2## where K: constant.
As is clear from equation (2), the limiting current Ip* is determined by the equivalent sectional area S of pores representative of the denseness of the gas diffusion layer and the thickness l of the gas diffusion layer.
As the thickness l of the gas diffusion layer increases, the limiting current Ip* decreases. But an excessively large thickness affects response speed and durability and therefore there exists an upper limit of the thickness. Accordingly, the limiting current Ip* essentially depends on the equivalent sectional area S of pores of the gas diffusion layer. The smaller the equivalent sectional area S, that is, the denser the gas diffusion layer, the smaller the limiting current Ip* becomes to meet effective detection control in the rich region.
A conventional stoichiometric sensor (O.sub.2 sensor) is disclosed in, for example, JP-A-13980 published on Feb. 8, 1978 which corresponds to a Japanese patent application by Suzuki et al filed July 23, 1976. The sensor in this literature has a gas diffusion layer prepared through plasma spraying process. The gas diffusion layer has a two-layer structure in which two layers have different thicknesses, with the outer layer closer to the exhaust gas having a thickness of 80 .mu.m and larger pores and the inner layer closer to the solid electrolyte having a thickness of 30 .mu.m and smaller pores.
A lean sensor is disclosed in, for example, U.S. Pat. No. 4,356,065 to Dietz issued Oct. 26, 1982. The patented sensor has a gas diffusion layer of a two-layer structure prepared through plasma spraying process. The thickness of a first layer having larger pores is 300 .mu.m and the thickness of a second inner layer having smaller pores is 2 nm.
The gas diffusion layers of the two known sensors are prepared through plasma spraying process. The plasma spraying process is one of excellent methods for formation of porous films but is disadvantageous in that:
(1) A prepared layer inevitably has a relatively large thickness and accordingly its response speed is poor and also weak in thermal stress in the operation;
(2) During spraying, the material is exposed to high temperatuers and cracks due to thermal stress tend to occur in the layer;
(3) Many sensors can not be manufactured at a time; and
(4) The manufacture cost is high.
Another sensor is known which does not rely on plasma spraying but uses a sintered material of superfine particles. This sensor corresponds to, for example, a gas sensor using superfine tin oxide particles described in National Technical Report, Vol. 26 (1980), page 457. The sintered material takes advantage of the fact that the surfaces of superfine particles are very active and highly adsorptive for gases, and is used as a solid electrolyte. However, this literature never teaches that the sintered material can be utilized for limitation of the diffusion speed of gases in the gas diffusion layer of the A/F sensor, such limitation being the task of the present invention, and the operation of the sintered material essentially differs from that of the gas diffusion layer of the A/F sensor.
In the past, superfine particles have also been used for a partition film for gas separation, as exemplified in "KOGYOZAIRYO", vol. 31, No. 7 (1983), page 50. The partition film for gas separation is adapted to extract a desired gas component from a mixture gas. This literature describes that gas separation takes advantage of the fact that when gases pass through a space whose size is smaller than a mean free path of the gases, the diffusion coefficient of the gases depends on the size of the space.
Exchangeability does not exist between this partition film for gas separation and the gas diffusion layer of A/F sensor to which the present invention pertains and the two are quite different from each other. While the pore diameter of the gas-separation partition film is 100 .ANG. or less, the pore diameter of the gas diffusion layer of A/F sensor to which the invention pertains measures 200 to 500 .ANG.. The gas-separation partition film is used in clean environments at room temperature but the A/F sensor is used in polluted envirnments at high temperatures. The former is applicable to selection of gas but the latter is applied to control of the speed of gas flow. Especially, because of different pore diameters, the partition film and the gas diffusion layer operate quite differently as will be described below.
When gases diffuse through pores, the gas flow takes the form of a molecular flow if a pore has a diameter smaller than a mean free path of the gases and the molecular flow has a diffusion coefficient D.sub.A indicated by, EQU D.sub.A =9.7.times.10.sup.3 d (T/M.sub.A).sup.0.5 ( 3)
where
On the other hand, if the pore diameter is larger than the mean free path of the gases, the gas flow takes the form of a viscous flow having its diffusion coefficient D.sub.AB which is given by, ##EQU3## where P: gas pressure
By substituting equations (3) and (4) into the previous equation (1), the diffusion limiting current for the molecular flow is, ##EQU4## where C.sub.1 : constant
and the limiting current for the viscous flow is, ##EQU5## where C.sub.2 : constant.
Thus, equations (5) and (6) demonstrate that dependency of the limiting current upon temperatures exhibits counter positive and negative correlations in accordance with the relation in magnitude between the pore diameter and the mean free path of the gases. In particular, when the pore diameter nearly equals the gas mean free path, the gas flow takes the form of a medium flow between the molecular and viscous flows and the limiting current becomes substantially unchangeable with temperature.
Because of a difference in pore diameter, the gas flow in the gas-separation partition film takes the form of a molecular flow but the gas flow in the gas diffusion layer takes the form of a medium flow. Accordingly, the output of a sensor using the gas diffusion layer depends on temperatures very little and this sensor is suitable for use as an A/F sensor. However, in contrast, since the output of a sensor using the gas-separation partition film remarkably depends on temperatures, this sensor is unsuitable for use as an A/F sensor.